Experimental Group Designs: Strengths & Weaknesses

Independent Group Designs

Here different participants are used between the different experimental conditions.

Example:

If for example you were testing how age affects memory recall you may have different groups of people of specific age ranges from one group to another. One group may have teenagers aged 12-19 while another group may have adults aged 25-35. Participants may be randomly selected based purely on age to insure fair allocation between the groups. The IV here may be the different age ranges with each group tested using the exact same conditions.

Another IV may see group members randomly allocated between different groups and then exposed to different IVs in the form of the experiment itself. For example one group may be shown pictures and words while another group is shown words only.

Strengths – Advantages

Participants can be randomly allocated to try address individual differences that may affect results.

Another strength is there are no “order effects” with the participants being exposed to the experimental condition only once unlike a repeated measures design where participants experience each condition one after another. Participants are also unlikely to be bored or tired in this design which may affect their results as they work through the different experimental conditions.

More participants tend to be used overall compared to repeated measures meaning the results may have more external validity to wider generalisation.

Independent group designs only need participants to complete the conditions once and if the different conditions are run at the same time this can result in a much quicker experiment than a repeated measures design which may be more time consuming.

Weaknesses – Disadvantages

The study requires more participants which may mean the study takes longer to gather them compared to a repeated measures design where you can re-use a smaller sample.

There may still be important significant individual differences between participants that may not be randomly filtered out through allocation resulting in skewed results. This may reduce internal validity and reliability of results.

Repeated Measures Design

A repeated measures design is when the same group of participants are used between the different experimental conditions.

Example:

You wish to test whether pictures and words help in recall vs only words alone. You have a group of 20 participants and test them in the first condition which is pictures and words and then you use the same group members to test them in the second condition which is words alone (different set of words but of similar difficulty/syllables etc). You then check the results.

Strengths – Advantages

One major strength is you require less group members as you simply re-use the same participants. This means the experiment is done much quicker than gathering participants between conditions like in an independent group design experiment.

Another advantage is there is no risk of individual differences affecting the results between the different conditions as participants are effectively comparing against themselves.

Weaknesses – Disadvantages

“Order effects” may occur where participants become better in the second condition due to practice from the first leading to improve or biased results.

Participants may also display “demand characteristics”if the experiment results in them guessing what the purpose of the study is. The design can not be used where they may “learn” from the first condition which may affect the results in the second.

Participants may become bored or tired between the two conditions possibly affecting the results in the second condition.

Matched Pairs Design

Separate groups are used but participants are matched between one another for similar characteristics or traits such as age, gender, IQ, nationality etc. This is to try and control for possible individual differences and stop them affecting the results.

Example:

You have two groups both being tested on memory recall. Both groups have participants that have been matched as closely to one another as possible based on relevant traits such as age, gender, similar background etc. The IV may be different methods of testing recall such as words only or words and pictures. The two groups would be tested with one group doing words only while the other group does pictures and words. As each participant is believed to be accordingly “matched” they compare the results to see which condition (words only or words and pictures) results in better recall.

Strengths – Advantages

Matching participants as close as possible on measurable traits may help control for individual differences affecting the results.

This design is better suited when a repeated measures design may not work due to an order effect occurring which may affect results.

Weaknesses – Disadvantages

Trying to match participants for similar traits is very time consuming.

Trying to match people exactly is impossible as there will always be individual differences from one person to another. Therefore the study may lack internal validity if these differences affect the IV rather than the experimental condition.

The sample may be smaller as trying to find a large sample of people matching across similar traits may be difficult. This may mean the findings may lack external validity.

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